The 90 is definitely a 4 stroke/mix, and it is a pro grade trimmer, not homeowner machine. Stihl recommends a one time valve adjustment at 139 hours, or as needed depending on how the machine is running.

The stihl 4 mix stuff I never adjust the valves the stuff runs good for years of heavy use which is why I like the brand. When I had the shin 4 mix trimmers they were each at the dealers half a dozen times that year. I didn't just run away I ran away from that stuff screaming. The shin 2 cycle is way better than their 4 mix but still lacks the endurance I demand in a handheld piece of equipment.